Not sure about the exact situations, but there is always a judgement call to be made: the tension between making progress and undue hesitation.
Junctions you can think of as "closed" or "open" - the former being where visibility is limited and you need to stop and look. The latter you can approach at a sensible pace while looking to flow onto the major road PROVIDING it's safe and doesn't cause anyone else to take avoiding action. As long as you're under control and can stop safely before the line if you need to. Plan ahead is the key.
Roundabouts are complex junctions. Normally you'd aim to flow onto them without stopping, but the usual rules apply regarding having enough space.
I'd always give way to both lanes when joining, as you never know when a vehicle on the roundabout is going to change lanes.
But the bigger issue seems to be the friction between the student and instructor. I've had some "challenging" instructors in my time, and I'm sure they'd say the same about some of their students. Either you're getting:
1. poor advice
2. poorly communicated good advice
... or a third option..
3. good advice, well communicated - but you're not listening
I don't think it's 3.
2. could be worth sticking around for and gritting your teeth.
1. would be a good time to shop around.
I have noticed some instructors seem focused on what you do wrong and come across as very critical. It's nothing personal, and if you can get past it there's usually plenty to learn. He won't bite your head off for asking to clarify.